"Existentialism is a doctrine that renders human life possible." Jean Paul Sartre: Existentialism is a Humanism

"Existentialism is nothing else but an attempt to draw the full conclusions from a consistently atheistic position." Sartre

Could any of you please explain existentialism to me? I've been learning about it in my philosophy class but I just don't quite get it yet. Something to deal with the analysis of individual existence and why you are who you are and what makes you that. How you define your own existence, what are the beginnings and ends of existence and how are they determined.

If someone would like to enlighten me on this, it would be greatly appreciate.

Thanks,
Will